Need help DCLCD 19" LCD Monitor - I already replaced caps

[DJM_PHX]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

I see the same voltage pulsing on what looks like a fuse (red). I also see a pulsing -85V on the metal piece framed in green. Isn't that a heat sink?

[retiredcaps]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

1) Blk 0-0.03V
2) Brn 0V
3) Red 0V
4) Org 0V
5) Yel 0-0.04V
6) Red 0-0.04V
7) Blk 0-0.04V
8) Yel 0V
9) Gry 3.5V
10) Wht ~0.002V
11) Blk 0V
12) Brn 0V
13) MT
14) Red 0-0.04V
15) Org 0-0.04V

Okay, a lot of voltages are zero for GND.

The large cap measured 172V-174V. Varying like the other voltages above.

If you are getting a fluctuating voltage at the big cap, this could be a sign that the big cap is bad. I noticed this board was made in Dec 2003 which means the caps are 7 years old. Let's assume the big cap is good for now, but note it for later.

The little cap that I referred to earlier comes into play after the big main cap.

Is the secondary voltages you are talking about the 2V variation?

The main (or primary) voltage is what comes from your wall. The term secondary refers to voltages that are not main. The LCD requires many voltages to operate the electronics. Hence the need for secondary voltages.

Since we are not seeing many secondary voltages in your measurement above, let's change out just small cap lying on its side next to the big cap. Normally, we recommend that you change all the electrolytic capacitors, but I'm trying to help you find root cause first. So by changing the little one, we might find that as root cause.

[jetadm123]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

1) Blk 0-0.03V
2) Brn 0V
3) Red 0V
4) Org 0V
5) Yel 0-0.04V
6) Red 0-0.04V
7) Blk 0-0.04V
8) Yel 0V
9) Gry 3.5V
10) Wht ~0.002V
11) Blk 0V
12) Brn 0V
13) MT
14) Red 0-0.04V
15) Org 0-0.04V


The large cap measured 172V-174V. Varying like the other voltages above.


Is the secondary voltages you are talking about the 2V variation?



I will have to try to borrow a dept camera to do this. I will try for today.
I have gone over the other pic and it is like mine, but I will follow up with the pic.
Number off of the board: LM004 Rev J 12/01/2003

A few questions:

1) Looks like the power supply is not running by the looks of the outputs at the wiring harness. When you took the voltage readings, was everything hooked up? Some power supplies require a load before they will start.

2) I circled a connector on your photo. Was it connected to anything?

3) The small cap that's next to the large cap. Try to measure the voltage across it. Please report what you see. Be careful of the high voltage cap next to it.

4) If you can post a photo of the entire bottom of the board, it would be helpful.

[retiredcaps]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

I see the same voltage pulsing on what looks like a fuse (red). I also see a pulsing -85V on the metal piece framed in green. Isn't that a heat sink?

F on the pcb denotes a fuse. A good fuse should measure less than 1.0 ohms (with power off and lcd unplugged).

The metal piece is a heatsink. -85V again means you have the wrong GND point.

Change out that little cap first and then we can proceed.

[kingofcoins]

Re: High Main Cap Voltage

This thread reminded me of an X20G Scepter monitor that I last summer read 375VDC on the main cap...yanked the plug and put it aside.
So for future reference, what if a main cap is quite elevated over the 160VDC level for LCD power supplies? In which ways should the repair tech proceed?

[DJM_PHX]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

A few questions:

1) Looks like the power supply is not running by the looks of the outputs at the wiring harness. When you took the voltage readings, was everything hooked up? Some power supplies require a load before they will start.

Everyting is/was connected. Double checked all of them.

2) I circled a connector on your photo. Was it connected to anything?

Nothing was connected there.

3) The small cap that's next to the large cap. Try to measure the voltage across it. Please report what you see. Be careful of the high voltage cap next to it.

11V-14V varies like the rest

4) If you can post a photo of the entire bottom of the board, it would be helpful.

I returned the dept camera. I can get it again tomorrow.

Also the heat sink. I had the black on the PCB screw connector (just right of the optoisolator) and touched the red to the sink. I would think I should not register anything, but I do. I also tried it with the black lead connected to the green ground wire on the screen frame. -80+V.

The fuse is good.
REV 3 not J on the board.

I can change them both out. The big and the little. The pulsing voltage looks to me like a cap building then leaking down over and over. I usually only ramp them until breakdown on our structures on wafers.

[retiredcaps]

Re: High Main Cap Voltage

This thread reminded me of an X20G Scepter monitor that I last summer read 375VDC on the main cap...yanked the plug and put it aside.
So for future reference, what if a main cap is quite elevated over the 160VDC level for LCD power supplies?

I believe the X20 has a PFC to get 375V DC and this one doesn't. So 160V DC is correct for this monitor.

[jetadm123]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

I can change them both out. The big and the little. The pulsing voltage looks to me like a cap building then leaking down over and over. I usually only ramp them until breakdown on our structures on wafers.

The small cap should be connected near a SOIC-8 PWM chip on the bottom of the board. Can you read off the part number of that chip?

[DJM_PHX]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

The small cap should be connected near a SOIC-8 PWM chip on the bottom of the board. Can you read off the part number of that chip?

SG3842GS
ADAX87140421E

Need to leave work until tomorrow.
Thanks for working with me on this. I will pull the caps tomorrow and get then replaced asap and test. If you have anymore checks for me to try, I will be checking the board.

What about the white paste that is on the caps to hold them to the board. Will I need to replace that?

[jetadm123]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

SG3842GS
ADAX87140421E

Need to leave work until tomorrow.
Thanks for working with me on this. I will pull the caps tomorrow and get then replaced asap and test. If you have anymore checks for me to try, I will be checking the board.

What about the white paste that is on the caps to hold them to the board. Will I need to replace that?

The white adhesive you're talking about is only used during the assembly process and does not need to be replaced.

[retiredcaps]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

I can change them both out. The big and the little. The pulsing voltage looks to me like a cap building then leaking down over and over.

I recommend changing them one at a time so we know root cause. Make sure to use low quality ESR caps like Rubycon MCZ, Panasonic FM/FC, or United Chemi-con.

[SMITHWHO]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

[QUOTE=SMITHWHO;141121]

dclcd 19" monitor turn monitor on boot up window log comes on. then goes black if i turn the monitor off then
on the screen comes on for second or two then black
if i turn the monitor off and then on screen comes on for
a second or two
if after many on and off trys it will come on and stay on
it will continue as long as you use it even with screen saver. but if you let it sleep then we are back at turn off and on I did find the large cap 100md 400 volt to be bad
I CHECKED EVERY THING SUGGEST BY YOU HELP PLEASE
sincerly j.r. Smith

my

[jetadm123]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

[QUOTE=SMITHWHO;142248]

dclcd 19" monitor turn monitor on boot up window log comes on. then goes black if i turn the monitor off then
on the screen comes on for second or two then black
if i turn the monitor off and then on screen comes on for
a second or two
if after many on and off trys it will come on and stay on
it will continue as long as you use it even with screen saver. but if you let it sleep then we are back at turn off and on I did find the large cap 100md 400 volt to be bad
I CHECKED EVERY THING SUGGEST BY YOU HELP PLEASE
sincerly j.r. Smith

my

That's unusual, since the large cap very seldom fails. It's usually the small caps that fail. What you're describing is a very good example of a problem caused by bad caps. While the caps you measured capacitance wise may have checked ok, they could still have a high ESR, which could cause the problems you're seeing. There is one small cap mentioned on the previous posts of this thread that if failing, can cause startup problems.

[retiredcaps]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

if after many on and off trys it will come on and stay on

Sounds very much like caps that exhibit high ESR. As the lcd warms up, the cap's ESR drops enabling the lcd to turn on.

Also, please find the "." on your keyboard. It makes your posts more understandable.

[DJM_PHX]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

Does 22uF 50V sound correct for the small cap? They had so much of the white goop on it that the printing mostly came off trying to clean it.

[alexanna]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

Does 22uF 50V sound correct for the small cap? They had so much of the white goop on it that the printing mostly came off trying to clean it.

Yes,Look at post #33 and #34
Al.

[DJM_PHX]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

Yes,Look at post #33 and #34
Al.

Thanks, missed that scanning thru.

[SMITHWHO]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

Needs del

[SMITHWHO]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

i checked all solder joints - they seems to be fine. I can not read if the little cap is 22uf 50v or it is 2.2uf 50v.

Does anybody know what it is?

its 22 mf 50 volts on my dclcd

[DJM_PHX]

Re: Need help DCLCD 19" LCD Monitor - I already replaced caps

SG3842GS
ADAX87140421E

I am waiting for my 22uF caps to come in so in the mean while, I was reflowing the solder on the connector and the transformers as you suggested to others.

On the SOIC-8 PWM chip, I found a single strand of copper wire that looked like it was laying between the two pins where the yellow line off the yellow circle is on the attached pic. I had it under a microscope and when so I brushed it off of the board. It was so small, I couldn't find it on the table. Maybe a failure mode?

I also measured the 22uF cap with a Fluke and it returned 21.7uF. Hmmm... I don't know about under a load though.